Many polymers, which are used for making various articles of manufacture by molding, extruding and the like processes, are difficult to paint, or at least it is difficult to prevent the paint on such articles from cracking or peeling. It is also difficult to get ink to adhere to films of such platics which are used for packaging. Numerous methods have been tried to obtain better paint adherence on such polymeric articles.
It is known that thiocyanogen is an active compound useful for treating the surfaces of polymers to enhance their adhesive properties, e.g. adherence of polymer films to each other or to paint applied to such polymer surfaces. Thiocyanogen is also useful in reacting with organic compounds to prepare organic thiocyanates or isothiocyanates.
The reaction with the polymers or with organic compounds is advantageously conducted in solution since the thiocyanogen is not normally isolable as such. Even in solution, however, the thiocyanogen is not stable and will precipitate from solution as a polymeric material which is unreactive for the purposes desired. Thus, these solutions must be used within a short period of time after preparation or prepared in situ. Processes of thiocyanating surfaces of polymers are described in U.S. Pat. Nos. 2,781,331; 3,607,536 and 4,567,241. Another method of thiocyanating polymer surfaces is disclosed by a different inventor in a copending U.S. patent application (Ser. No. 13,487, filed Feb. 11, 1987) in which the surface is treated with a stabilized thiocyanogen. The disclosure teaches that (SCN).sub.2 in combination with certain .alpha.,.beta.-diketones, such as o-chloranil, forms a stable composition which can be used to obtain the same results as when treating the surface of polyurethanes with the unstabilized thiocyanogen. The method and compositions disclosed therein are incorporated herein by reference.
Current methods in use industrially, however, for promoting adhesion on difficultly paintable polyurethane reaction injection molded (RIM) parts usually involve washing the surface with a multi-stage wash, using acid, e.g. phosphoric acid, and a detergent, but such treatments render the surface paintable for only a few hours and are not as effective as the above indicated use of thiocyanogen.
The present invention is a method of treating the surface of a polymer which does not employ thiocyanogen, but is equally effective in modifying its surface to improve paint and/or ink adherence.